This invention relates generally to the field of geophysical prospecting. More particularly, the invention is a method of identifying structural and stratigraphic discontinuities in a three-dimensional (3-D) seismic data volume.
As part of the hydrocarbon exploration and production work process, geoscience interpreters often need to recognize and map subsurface structural features, such as faults, and stratigraphic features, such as channel- or sand-body edges, in three-dimensional seismic data. However, identifying structural and stratigraphic features in 3-D seismic data can be a time consuming, subjective, and difficult process. There is a need to generate, in a computationally efficient matter, a derivative data volume (i.e., a data volume derived from the original seismic data volume), that displays clear sharply focussed structural and stratigraphic features that can be quickly recognized and exploited in the mapping process.
Several techniques have been used in the oil industry to enhance the interpretation of structural and stratigraphic features in 3-D seismic data. A well-known technique is to transform the original amplitude data into a coherence volume using a series of one-dimensional cross-correlation calculations. For every data sample in a volume, the cross-correlation calculation is performed using a user-defined vertical window with the equivalent portion of an adjacent trace. Typically, the vertical window is the target sample in question, plus 3-7 data samples above and below the target sample, depending upon the frequency of the data. This operation is repeated for all data samples and all traces, all in the same correlation direction. The correlation direction is generally in-line, cross-line, or either diagonal direction. The resulting coherence volume typically contains values normalized between xe2x88x921 and +1. For adjacent traces that are very similar, the value of the coherence sample will be close to +1, since +1 represents high correlation. This similarity, and hence correlation, is expected for adjacent traces that do not straddle a structural or stratigraphic discontinuity. For adjacent traces that do straddle a discontinuity, lack of similarity is expected. Thus, their coherence value would be closer to 0, since 0 represents no correlation. A coherence value of xe2x88x921 represents negative correlation, such as high correlation with phase reversal. This standard technique has limitations, though, because features perpendicular to the single correlation direction are highlighted, while features parallel to the correlation direction are poorly imaged.
Bahorich and Farmer received U.S. Pat. No. 5,563,949, xe2x80x9cMethod of Seismic Signal Processing and Explorationxe2x80x9d, issued Oct. 8, 1996. This patent is commonly known as the xe2x80x9ccoherence cubexe2x80x9d patent. Bahorich and Farmer also obtained a continuation of this patent in U.S. Pat. No. 5,838,564, xe2x80x9cApparatus for Seismic Signal Processing and Explorationxe2x80x9d, issued Nov. 17, 1998.
Bahorich and Farmer""s ""949 patent describes a method for converting a fully processed 3-D seismic data volume into a cube of coherence measurements. According to their method, the 3-D data volume is divided into a plurality of horizontal slices, and each horizontal slice is further divided into a plurality of cells, each of which contains portions of at least three seismic data traces. As described in the ""949 patent, these at least three traces in each cell comprise a reference trace, an in-line trace, and a cross-line trace. The in-line trace and the cross-line trace are each compared to the reference trace in each cell using a measure of coherency. Then the in-line and cross-line coherency measures are combined to obtain a single value that is representative of the coherence of the three seismic traces for each cell. This process is repeated for every cell, using every trace in the 3-D seismic volume as a reference trace, in order to obtain a 3-D cube of coherence measurements. Bahorich and Farmer""s ""564 patent describes the corresponding apparatus for carrying out the process of their ""949 patent.
Bahorich and Farmer""s patented technique combines information from more than one correlation direction at each data sample in the 3-D seismic data volume, thereby highlighting structural and stratigraphic information along multiple azimuths. According to Bahorich and Farmer, in their invention xe2x80x9cthe concept of cross-correlation is extended to two dimensions by taking the geometric means between the classical one dimensional cross-correlationsxe2x80x9d (U.S. Pat. No. 5,563,949, column 4, lines 17-20). This technique has limitations, however. Combining information from different correlation directions may effect the image clarity of the structural and stratigraphic features. This decrease in clarity can make it more difficult to extract structural and stratigraphic information in automated mapping processes. In addition, the computational complexity of this procedure is significantly greater than the traditional method using classical one-dimensional cross-correlations.
Higgs and Luo received U.S. Pat. No. 5,724,309 xe2x80x9cMethod for Geophysical Processing and Interpretation Using Instantaneous Phase and Its Derivatives and Their Derivativesxe2x80x9d, issued Mar. 3, 1998. Higgs and Luo""s ""309 patent describes a related technique for interpretation of faults and stratigraphic features. The technique uses instantaneous phase and its spatial derivatives to determine values of spatial frequency, instantaneous frequency, dip magnitude and dip azimuth. These values are plotted to produce a derivative seismic volume that highlights subsurface changes. The main advantage is its computational speed. However, the instantaneous phase and frequency images tend to be of lower resolution than traditional cross-correlation images. A similar technique was also published by Hardage et al., 1998, xe2x80x9c3-D Instantaneous Frequency used as a Coherency/Continuity Parameter to Interpret Reservoir Compartment Boundaries Across an Area of Complex Turbidite Depositionxe2x80x9d, Geophysics, Vol. 63, No. 5, pp. 1520-1531. This technique uses instantaneous frequency images to define reservoir compartments by identifying facies boundaries.
Gersztenkom""s International Patent Application No. PCT/US97100249, xe2x80x9cMethod and Apparatus for Seismic Signal Processingxe2x80x9d, was published as International Publication No. WO 97/39367 on Oct. 23, 1997. This technique generates a covariance matrix for an ensemble of seismic traces and then estimates the degree of similarity between traces by estimating the largest eigenvalue of the covariance matrix. It identifies the maximum coherence component and therefore identifies structural and stratigraphic discontinuities in the data at all azimuths. The main disadvantage is that because this technique estimates eigenvalues of the covariance matrix for each time sample in the volume, it is computationally intensive.
Marfurt, Kirlin, Farmer, and Bahorich received U.S. Pat. No. 5,930,730 xe2x80x9cMethod and Apparatus for Seismic Signal Processing and Explorationxe2x80x9d, issued Jul. 27, 1999. The ""730 patent describes a method for identifying structural and stratigraphic features in three dimensions. After datumming is applied to remove a significant portion of the regional structural dip, a semblance calculation is applied as a function of time to multiple seismic traces in multiple directions to further estimate and correct for local dip. A maximum semblance cube is created that highlights structural and stratigraphic discontinuities, corrected for structural dips. Improved imaging is obtained in areas of higher structural dip and seismic noise. The main disadvantage of this method is that it is very computationally intensive.
Marfurt, Gersztenkorn, Nissen, Sudhaker, and Crawford published a paper in Geophysics, Vol. 64, No. 1, pp. 1040111, January-February 1999, xe2x80x9cCoherency Calculations in the Presence of Structural Dipxe2x80x9d. The technique described in this publication examines the similarity of multiple traces at various time lags to estimate the dip of reflectors. An eigenvalue algorithm is then used to calculate the similarity of traces in the locally averaged dip direction. The main advantage of this approach is the minimization of coherency artifacts due to the dip of reflectors and thus a sharpening of the image. The main disadvantage is that this approach can be computationally intensive.
It can be seen from the foregoing that a need exists for a computationally efficient method for identifying structural and stratigraphic features in 3-D seismic data that effectively images features of different orientation directions while maintaining image clarity.
The present invention is a method for detecting structural and stratigraphic discontinuities in a 3-D volume of seismic data samples. One embodiment comprises the following steps. First, a plurality of directions in the 3-D volume are selected in a sequential order. Next, a series of sequentially less restrictive thresholds is defined. Then, the following steps are performed for each data sample in the 3-D volume until the sample has a value stored at the corresponding sample location in the output discontinuity volume. First, one-dimensional, two-trace discontinuity values are calculated for the data sample sequentially along the directions and the first of the calculated discontinuity values that satisfies the first threshold is stored in the output discontinuity volume. Then, the following steps are repeated for the data sample until the sample has a value stored at the corresponding sample location in the output discontinuity volume. First, the next less restrictive threshold in the series of thresholds is selected. Then, the discontinuity values calculated along the directions are compared sequentially to the selected threshold and the first of the discontinuity values that satisfies the selected threshold is stored in the output discontinuity volume.
A further embodiment of the present invention comprises the following steps. First, a plurality of directions is selected containing a primary direction and at least one secondary direction. Next, one-dimensional, two-trace first discontinuity values are calculated along the primary direction for each seismic data sample in the 3-D data volume. Next, a series of sequentially less restrictive thresholds is defined, such that a significant portion, preferably at least approximately 10%, of the first discontinuity values satisfy the first threshold. This significant portion of first discontinuity values is then stored in an output discontinuity volume at the corresponding sample locations. The following steps are then repeated for each remaining data sample until that sample has a value stored at the corresponding sample location in the output discontinuity volume. First, one-dimensional, two-trace discontinuity values are calculated for the sample sequentially along the secondary directions and the first of the calculated discontinuity values that satisfies the first threshold is stored in the output volume. If none of the calculated discontinuity values satisfies the first threshold, then the next less restrictive threshold in the series of thresholds is selected. Finally, the discontinuity values calculated along the primary and secondary directions are compared sequentially to the selected threshold and the first of the discontinuity values that satisfies the selected threshold is stored in the output discontinuity volume. This process is repeated until a discontinuity value has been stored in each sample location in the output discontinuity volume.
A further embodiment of the present invention comprises the following steps. First, second, third and fourth directions are selected in a 3-D volume of seismic data samples. First, one-dimensional, two-trace discontinuity values are calculated along the first direction for each sample. A primary threshold and a series of sequentially less restrictive secondary thresholds are defined such that a significant portion, preferably at least approximately 10%, of the first discontinuity values satisfy the primary threshold. Then, this significant portion of the first discontinuity values is stored in an output discontinuity volume at the corresponding sample locations. The following steps are then performed for each remaining data sample, until the sample has a value stored at the corresponding sample location. A second discontinuity value is calculated along the second direction for the sample and is stored in the output volume if it satisfies the primary threshold. If the second discontinuity value does not satisfy the first threshold, then a third discontinuity value is calculated along the third direction for the sample and is stored in the output discontinuity volume if it satisfies the primary threshold. If the third discontinuity value does not satisfy the first threshold, then a fourth discontinuity value is calculated along the fourth direction for the sample and is stored in the output discontinuity volume if it satisfies the primary threshold. Finally, the following steps are repeated until the sample has a stored value. The next less restrictive threshold in the series of secondary thresholds is selected. The first discontinuity value is stored in the output discontinuity volume if it satisfies the next threshold. If the first discontinuity value does not satisfy the next threshold, then the second discontinuity value is stored in the output discontinuity volume if it satisfies the next threshold. If the second discontinuity value does not satisfy the next threshold, then the third discontinuity value is stored in the output discontinuity volume if it satisfies the next threshold. If the third discontinuity value does not satisfy the next threshold, then the fourth discontinuity value is stored in the output discontinuity volume if it satisfies the next threshold.